Processes for electrodepositing metal in various thicknesses on substrates, are well known in the art. Typically, an electroplating bath comprising ions of the metal to be deposited and a suitable electrolyte is provided. The article or object to be plated is immersed in or otherwise contacted with the bath while connected as the cathode to an external current source, and a metal electrode is connected as the anode to the same current source. During operation, ions of the metal to be deposited are reduced in the bath to zero valance metal which plates out on the workpiece surface.
Special mention is made of methods for electrodepositing metallic palladium on substrates, particularly metallic surfaces. In such cases, the palladium deposition bath tends to be unstable and cannot be used continuously for extended periods without undergoing significant losses in bath efficiency. The term "bath efficiency" herein refers to the comparison at a given current density between the actual bath plating rate and the theoretical bath plating rate as determined mathematically from Faraday's Law.
Although conventional palladium electrodeposition baths may display good efficiency initially and shortly after plating has begun, this efficiency often decreases sharply within a few hours, and in some cases dropping off to less than 50% of the original value after only about twenty-four hours of continuous use. To maintain the palladium plating rate at or reasonably near original levels, it is usually necessary to supply a higher current to the plating bath, a procedure which will produce unsatisfactory electrodeposits.
Without wishing to be bound by any theory, it is believed that the palladium ions in the bath are oxidized to higher valence states during operation, thus making it more difficult to reduce the palladium ions to metallic palladium and plate out without supplying more current to the bath. The problem of unstable plating efficiencies is recognized in the art. For example, U.S. Pat. No. 4,092,225 states that a rapid decrease in cathode efficiency occurs at current densities in excess of about 4 amps per square decimeter. In additional published data on palladium plating systems, the development of erratic cathode efficiencies as the plating bath ages during use is described. See "Precious Metal Plating" by Fischer and Weiner (1964) Page 203 and U.S. Pat. No. 1,779,436.